Half-moon hairstreak (Satyrium semiluna) COSEWIC assessment and status report: chapter 10
Limiting factors and threats
The presence of the larval food plants is thought to be one of the primary limiting factors of the butterfly at the landscape scale. The species is associated with warm, dry habitats and is at the northern extent of its range in Canada, thus suggesting regional climate as a limiting factor. The presence of the appropriate species of ants may also be a limiting factor. Wildfire is potentially a local factor in habitat suitability and population size over time.
The Alberta population of S. semiluna is restricted to an alluvial fan where there is a strong likelihood of mortality of individuals from natural flooding. Some limitation to the population may also result from the mechanical effects of trampling by elk (Cervus canadensis) wintering in its habitat. The massive population reduction witnessed in 2004 suggests that extirpation through natural stochastic process, for example, unseasonable frost, is possible. The extirpation of a subalpine population of the Silvery Blue (Glaucopsyche lygdamus) and the severe reduction of populations of several other butterflies by an unseasonably late frost occurred in Colorado; the populations had not recovered several years later (Ehrlich et al. 1972). The only apparent and substantive human-induced threat to S. semiluna in Alberta is habitat change resulting from the expanding spotted knapweed (Centaurea maculosa) population and subsequent knapweed control activities. Knapweed is known to cause significant ecological damage by changing the structure and diversity of plant communities through competition for resources and release of allelopathic compounds (e.g., Tyser and Key, 1988; Kelsey and Locken, 1987). Knapweed has also been linked to increases in surface water runoff and soil sedimentation (Lacey et al. 1989). Control measures for knapweed, such as herbicide use, may also affect non-target plants in the area. The specific consequences for the Half-moon Hairstreak have not been researched, but such fundamental changes in the ecosystem are likely to have negative consequences for the butterflies, and so this weed is considered to be a significant threat to the Alberta population. Knapweed control is underway by Parks Canada. This is being done with due diligence and with regard for the undesirable effects of the control measures themselves, but even a combination of spot herbicide application and hand pulling results in some unavoidable damage.
The British Columbia population is subject to a wide variety of human activities that could depress or extirpate local populations. Livestock grazing is the most pervasive economic land use and is a likely factor in population dynamics on lands subjected to extremely intense and persistent grazing. Populations of Half-moon Hairstreaks occur on grazed lands, and the Richter Pass population co-exists with fairly heavy livestock grazing. However, in the White Lake basin, the butterflies were found in an area of light grazing, but none were seen just across the road where the same habitat had been very heavily grazed. Excessive livestock grazing represents a clear, existing threat, especially given that most known populations occur on private land where there are no legal requirements to prevent excessive livestock grazing. The amount of habitat at one of the Richter Pass locations may be reduced in the future through the expansion of an aggregate pit. Mining activity is also a potential source of habitat reduction. The human population is growing at a rapid rate in the area and this is expected to continue. Associated with increases in the human population, residential housing and road construction continues to consume habitat on Anarchist Mountain (Kondla, pers. obs. 2003), and housing expansion is very much an ongoing threat in the south Okanagan area. Native habitat is also being converted to intensive agricultural uses, including vineyards which are undergoing rapid expansion in the area. Knapweed was not observed in the occupied Half-moon Hairstreak sites in 2003. Lack of historical information precludes proof that any populations of the Half-moon Hairstreak have been affected by knapweed in the past, but knapweed and other weeds, are likely potential threats to extant populations of the Half-moon Hairstreak in its British Columbia range.
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